Oh.. bummer, I had a good post on that that has been wiped. I'll repost it here...

There were a couple threads... in one I had these instructions:

Hmm... so this is a case where you were using e-commerce on a site, removed those modules, and then installed Ubercart on the same site? I'm not entirely sure what issues that would create, but I can offer a little bit of help on a guess. This is a process that should remove any remains of the systems and give you a "clean" install:

1. Make a backup of your directory and your database!
2. Go ahead and remove Ubercart again.
3. From your admin interface, remove any product nodes you have created. Remove any taxonomy vocabularies related to e-commerce or Ubercart.
4. Clear all your cache tables. So anything called cache or cache_* just truncate. (On phpMyAdmin you do this with the Empty button.)
5. In the node_type table make sure there are no e-commerce or Ubercart entries hanging around.
6. Remove e-commerce and Ubercart related rows from the sequences table.
7. Scan through the variable table and remove any rows related to e-commerce and Ubercart. For Ubercart, it should be anything prefixed with uc_, but I can't vouch for the e-commerce variables.
8. Scan through the system table and make sure there aren't any rows hanging around from the e-commerce or Ubercart modules.
9. Try the install again and hope for the best!

And +1 on your idea to have renaming on product classes.

(Please note: the ideal situation is to not attempt to install Ubercart over e-commerce. It's just difficult, and because these modules are both large packages, something will probably slip through the cracks and not work totally right.)